ΔE 26.5, loose: different enough that you should pick on purpose. Pearl Green is noticeably lighter, much more saturated than Necron Compound. They are different kinds of paint: Citadel Necron Compound is a metallic and Tamiya Pearl Green is a pearl paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Pearl Green is noticeably lighter, much more saturated than Necron Compound. They are different kinds of paint.
The same difference applies the other way round: Necron Compound is noticeably darker, much more muted.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Necron Compound on the colour wheel See Pearl Green on the colour wheel
All matches: Necron Compound equivalents · Pearl Green equivalents